# balloons

Dear media,

The density of air is just over 1.2 kg per cubic meter. At about STP.
The density of helium is just under 0.2 kg per cubic meter.

A helium balloon can therefore lift about 1 kg per cubic meter.
There are about 30 cubic feet per cubic meter and just over two pounds per kg.

To lift a 50 pound load you therefore need about 750 cubic feet of helium filled balloon,
bit more allowing for envelope, rope, basket etc.

You do the math.

I hope they find that little kid in Colorado, and that he is very scared of his parents being angry with him.

This is allowing for buoyancy only, with wind a balloon can lift more.
How much more depends on the wind speed and the balloon shape.

PS: balloons are not aircraft; if they are buoyant then they are not "helium powered";
helium balloons are not the same as "hot-air" balloons.
Try especially hard not to make all these mistakes in the same paragraph.
It is not the lack of knowledge, it is the blatant inconsistency.
Sanity checks, folks.

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Pardon my ignorance, but what is "STP"?

The area they are searching is about 5000 feet above sea level (if you trust both the news channel and my memory); is that the density you used, or how does it change things?

Please look up "aircraft" in the dictionary.

STP is standard temperature and pressure, I think.

If the things that you post about are not enough to raise your eyebrows, read the info on the family. Parents are storm chasers, were 'stars' on a 'Trading Spouses' 'reality' TV show, and they have a psychic working with them that claims to speak with the dead. This kid was not experimenting, he was escaping!

The Woo, it burns!

(sorry about the liberal use of 's)

Your wish is granted. The kid was found, hiding.

I'm still curious about the math regarding altitude, though.

The Flight of the Falcon, on cable TV,
Was followed by millions (including, yes, me),
Who watched as the media chased a balloon
And hoped against hope that theyâd find the kid soon.

You ask why a knowledge of science is needed?
The info was there, though it wasnât much heeded:
The size of the craft was decidedly small,
And it couldnât have lifted young Falcon at all.

At five feet in thickness, and twenty feet wide,
The saucer held 600 cubed feet inside;
A hundred and fifty (or more) feet too few,
So flying was something the boy could not do.

(I cynically picture some geek on the staff
Who ran through the numbers and had a quick laugh,
Alerted the bosses: âthereâs nothing to fear!â
â--But the ratings!â they said, âget your ass outta here!)

I realize, just now, at the end of my verse,
I really canât figure which option is worse!
A cynical network, just jerking our chain,
Or science too tough for the news to explain!

http://digitalcuttlefish.blogspot.com/2009/10/flight-of-falcon.html

How can wind affect the lift of a balloon? Once it is launched it is at rest in its airmass and only serious windshear can affect it.

By Keith Harwood (not verified) on 15 Oct 2009 #permalink

The balloon is reported to be 20 feet wide and judging from the photos, has a height of maybe 5 feet. So if we consider a cylinder with radius of 10 feet and height 5 feet, I find a volume of 1570 ft^3. An oblate spheroid with the same dimensions has a volume of 1050 ft^3. So it seems to be roughly enough helium, to order of magnitude. (Though perhaps there are better numbers out there for the volume.)

See also the story of Larry Walters.

By Brian Lacki (not verified) on 15 Oct 2009 #permalink

I haven't been following, but I did see one picture, that made the balloon look quite a bit larger than the person beneath it. The cube root of 750 being about 9, I could imagine that 750 cubic feet is not all that much bigger than the balloon in question. I presume you chose 50 pounds because it's close to the boy's weight? Or is it meant to be a ridiculously small amount?

The quoted size if the ballooon was decidedly on the small size to lift a six year old.
Fifty pounds is on the light side for a six year old male in the US.

I used STP (Standard Temperature and Pressure) rounded down, because that gave an upper estimate for the buoancy. At 5000 feet the density is lower and the buoancy lower also.
The density decline is exponential but with a scale height of about 18,000 feet, so density there is not very different from sea level.

Lift can carry a balloon a long way, if the wind is high enough - just think of air balloons being carried off, or indeed cows during hurricanes... the live footage I saw suggested little wind at ground level. I did not think the balloon plus load being carried that way by wind lift or ram pressure was plausible with net negative buoyancy.

I posted after they found the kid was not onboard, but while it was being speculated the wooden box had fallen off early in the flight, with the kid in it.
I'd have felt awful if the kid had in fact been on board, but the numbers did not really allow for the kid having been in the balloon. Not given the weight of the balloon itself and the box under with ropes etc.

@Robert - ok, there are dictionaries that include dirigibles and general buoyancy lifted unpowered craft as aircraft.

We now have a video of the launch. The shape of the balloon did not change when it left the ground, which suggests that there was no significant weight being supported by the ground prior to launch. Certainly a 50-pound weight would cause an obvious change in shape if the balloon was suddenly forced to carry the weight.

http://thelede.blogs.nytimes.com/2009/10/16/authorities-investigating-f…

I may have gotten the physics all wrong! The ground would not necessarily have been supporting any weight prior to takeoff.

Probably not much applicable in this case, but as air density decreases with altitude, external pressure decreases and the balloon expands. You see high altitude balloons leaving the ground slack so they don't pop at altitude. I wonder how, or if, the relatioship between internal density and external density changes with altitude.

By Jim Thomerson (not verified) on 17 Oct 2009 #permalink

just discovered The Iceland Weather Report...nice

By ragnhildur (not verified) on 17 Oct 2009 #permalink

Clearly the media never saw the mythbusters where they tried to lift a four year old with helium balloons... it took 3500 to lift her a few feet. I wondered when the story came out.